Synthesis of no-carrier-added C-11 labeled D- and L-enantiomers of phenylalanine and tyrosine for comparative PET studies

Abstract The natural amino acids tyrosine (Tyr) and phenylalanine (Phe) were labeled with carbon-11 via a modified Bucherer-Strecker synthesis. A rapid reaction of the sodium bisulfite adduct of the aldehyde precursor with ammonia provided the precursor for radiosynthesis. [ 11 C]Cyanide displacement followed by base hydrolysis afforded the corresponding 11 C-labeled amino acids. The purification and chiral separation were simply achieved by using a combination of solid-phase extraction and chiral HPLC to afford individual enantiomers of each amino acid. The decay corrected radiochemical yields for each of the enantiomers were 12–16% with respect to the [ 11 C]cyanide after 40–45 min of radiosynthesis. Radiochemical purity of the products was >97% (typically >99%), enantiomeric excess was >98% with the specific radioactivity 2–3 Ci/μmol at the end of bombardment. Because of its simplicity and wide applicability, the described procedure could be the method of choice to produce [ 11 C]amino acids for PET studies.